I don't believe it is required, but I think it is a very good idea to have some coverage, especially for evacuation for trekkers in the event of serious injury or illness.

Source: US govt page on Nepal entry requirements for Americans traveling there.
https://travel.state.gov/content/tra...es/Nepal.html/

Most nationalities do require a visa on arrival, however. When we went last year, it was easy to fill out the paperwork online before traveling, then finish the process on site and pay the small fee. They did have trouble finding my husband's application (but not mine) in spite of applying one after the other on the same computer at the same time...